The expression of virulence determinants is tightly regulated so that they are produced at the right place and for the correct time during infection of an animal or plant host. The master regulator of Salmonella pathogenicity is the PhoP/PhoQ system, which consists of the sensor for extracytoplasmic Mg2+ PhoQ and the DNA binding transcriptional regulator PhoP. We have determined that PhoP-dependent genes can be differentially expressed in response to additional signals, which are often detected by the leader regions of PhoP-dependent mRNAs. This proposal describes experiments that explore: first, the identity of the signals detected by the leader sequence of the mgtCBR operon, which encodes proteins involved in Mg2+ homeostasis and virulence; second, the mechanisms by which the leader sequences of the mgtA and mgtCBR genes control expression of the corresponding coding regions; and third, novel PhoP-dependent genes that are regulated after the initiation of gene transcription step. An accomplishment of these goals will take us closer to understanding the mechanisms by which a microbe can modify its gene expression program in response to environmental cues as well as intracellular signals. Our project addresses the defining character that distinguishes bacteria and archeae from organisms that possess a nucleus: the coupling of transcription and translation. Therefore, it is anticipated that our findings will provide valuable insight into gene regulation in other bacterial species and by other regulatory systems. PUBLIC HEALTH RELEVANCE: Bacterial pathogens exert tight control over their virulence determinants. This proposal examines how this control is mediated by the leader region of certain mRNAs, which can respond to a variety of chemical and physical signals by modifying whether the corresponding coding regions are expressed.